C57BL/6 (B6) mice are genetically resistant to fatal HSV encephalitis (HSE) compared to 129S6 (129) and BALB/c that are susceptible. We have shown that fatal HSE in 129 mice results from hyper-inflammatory responses involving macrophages and neutrophils and that acyclovir which inhibits HSV replication is largely ineffective in protection against HSE. B6-Rag mice lacking B and T cells are more resistant than 129-Rag that are only slightly more susceptible than 129 wild type (wt) mice. We report that a single dose of pooled human immunoglobulin (IVIG) given at 24 h post infection (PI) completely protected wild type 129 mice from fatal HSE. IVIG protected virtually all B6-Rag mice, while 129-Rag mice were not protected even when given multiple doses of IVIG or IVIG supplemented with acyclovir (ACV). We suggest that the mechanism(s) of IVIG protection differs fundamentally in B6 and 129 mice and further, that a cell type absent in Rag mice is required for long-term protection akin to that seen in IVIG treated wt 129. We propose a two-component model for IVIG protection involving a mechanism to suppress inflammation and a mechanism to suppress HSV replication in neurons;both mechanisms are operative in 129 mice whereas only the second mechanism is operative in B6 mice. We hypothesize that IVIG activates a 'sensor'DC that in turn modulates innate effector cells (macrophages and monocytes) either directly of via induced regulatory T cells (Tregs) to achieve controlled inflammatory responses that control infection without bystander immune pathology. Our Specific Aims are designed to test the hypothesis proposed to explain IVIG's anti-inflammatory activity. The mechanism whereby IVIG suppresses HSV replication will be studied in a separate application. In Aim 1, the inflammatory response to HSV will be characterized in untreated and mice treated with IVIG or its derivatives (sialylated IgG, antibody subclasses, etc). IVIG effects on survival, virus load, lesion formation will be assessed and additionally, biodistribution of IVIG will be compared in mock and HSV infected WT 129 and 129/B6-Rag. In Aim 3, the IVIG 'sensor'cells (sDC) will be identified using an adoptive transfer strategy, it's role in protection will be investigated, including induction of Tregs and modulation of expression of Fc?Rs, cytokines, chemokines and their receptors in hematopoietic cells to define the anti-inflammatory mechanism(s) of IVIG in this model of viral encephalitis caused by excessive inflammation. Results from studies done thus far suggest IVIG will be useful for treating HSE and other immune mediated HSV diseases such as herpes stromal keratitis (HSK). Results from studies proposed here will uncover the mechanisms of suppression of neuroinflammation revealing the potential for application of IVIG to treat a broad spectrum of CNS inflammatory diseases. PUBLIC HEALTH RELEVANCE: Herpes simplex virus (HSV) is widely distributed in the human population as result of its ability to cause latent infections and subsequently reactivate leading to further spread. HSV is the major cause of sporadic encephalitis, which despite treatment with antiviral drugs is still associated with high mortality (~20%) and serious neurological sequelae in many survivors. HSV infections are particularly dangerous in immunosuppressed individuals and neonates with compromised immunity. We have shown convincingly that fatal HSV encephalitis (HSE) in susceptible 129S6 mice results from pathogenic inflammatory responses. Most importantly, treatment of 129 infected mice with pooled human IgG (IVIG) totally protected the mice from death. IVIG treatment also protected >80% of resistant C57BL/6 (B6) Rag mice that lack B&T cells but it failed to protect 129-Rag mice even when multiple doses of IVIG or IVIG supplemented with acyclovir (ACV) were given. This unexpected result shows that genetic factors determine the efficacy of IVIG treatment and our unique model will allow us to dissect the mechanism(s) of IVIG protection in mice with different genetic backgrounds, including Rag mice representative of immunocompromized individuals. Results from these studies will be important for making the case for IVIG as a treatment for neuroinflammatory diseases including encephalitis caused by HSV and other viruses like West Nile virus that are potential bioterrorism agents.